Improving the polymerization activity of catalysts in olefinic polymer production leads to the reduction in production costs, for which various studies have heretofore been made.
For example, H. H. Brintzinger (J. Polym. Sci.: Part A, Polym. Chem., Vol. 33, 1305-1317 (1995)) and S. Kojoh, et al. (Polymer, Vol. 36, No. 26, 5015-5018 (1995)) say that the activity of catalysts could be increased by controlling the hydrogen concentration in the polymerization system to fall between 3 and 20 mol % or so. Regarding its effect, however, hydrogen added could increase the catalyst activity at most 1.7 times or so, as compared with that in the absence of hydrogen.
On the other hand, known is a method of adding hydrogen that serves as a chain transfer agent, to the polymerization system in olefinic polymer production so as to control the molecular weight of the olefinic polymers produced. The method is employed in industrial-scale olefinic polymer production for which the catalyst used is typically an Mg--Ti-based, composite-type solid catalyst, and also in olefinic polymer production for which is used a metallocene-type catalyst being widely noticed in the art.
For example, in Japanese Patent Laid-Open No. 184179/1994, hydrogen is added to the olefin polymerization system for the purpose of controlling the molecular weight of the olefin polymer produced and/or enhancing the activity of the catalyst used. In its Examples, a metallocene-type catalyst having a single-crosslinked ligand of substituted indenyl groups as crosslinked at their 1-position is used for propylene polymerization, and hydrogen is added to the polymerization system whereby the activity of the catalyst used is enhanced. Regarding its effect, however, hydrogen added could increase the catalyst activity at most 2 times or so, as compared with that in the absence of hydrogen.
In International Patent Laid-Open No. WO96/30380, it is written that the activity of the catalyst used increases with the increase in the amount of hydrogen added to the polymerization system.
In the techniques noted above, the amount of hydrogen to be added to the polymerization system is large, relative to the amount of the olefin to be polymerized. In those, however, the effect of hydrogen added for activating the polymerization system is not large, and the absolute value of the polymerization activity could not be enlarged so much.
Given that situation, the object of the present invention is to provide a method for producing olefin polymers in which the polymerization activity of the catalyst used can be enhanced in a simplified manner and the catalyst used can therefore exhibit high activity.